The present invention relates to two-pack acrylic adhesive formulations and more particularly to novel catalyst systems therefor.
The use of structural adhesives has increased substantially in recent years in the aircraft, housing and construction, automotive and footwear industries, to name but a few large quantity users of such adhesives. The trend to structural adhesive formulations has been facilitated by the replacement of conventional metal and ceramic materials by polymers and by the development of a new generation of adhesives which are tough and can be applied quickly without burdensome surface preparation. Unlike conventional epoxy adhesive formulations which generally cure slowly and required careful mixing and surface preparation techniques, the second generation acrylic adhesives contain catalysts and promoters which promote rapid, room temperature cure and obviate the need for meticulous surface preparation and proportional mixing of components. Moreover, such second generation acrylic adhesive formulations possess excellent strength which is derived through special toughening mechanisms.
A variety of catalyst systems have been developed for reactive acrylic adhesive formulations. For example, prior catalyst systems include the combination of a tertiary amine and a peroxide (U.S. Pat. No. 3,333,025); aldehyde/amine condensation products in combination with certain chelates (U.S. Pat. No. 3,591,438); amines plus benzoyl peroxide (U.S. Pat. Nos. 3,725,504 and 3,832,274); a redox system of cobalt napthanate and MEK hydroperoxide (U.S. Pat. No. 3,838,093); the combination of amines, salts of transition metals, amine-aldehyde condensation products, and peroxides (U.S. Pat. No. 3,890,407); amines and peroxides in combination with oxidizable metal ions (U.S. Pat. No. 3,994,764); saccharin or copper saccharate and para-toluene sulfonic acid (U.S. Pat. No. 4,052,244); and peroxides and amine-aldehyde condensation products with a non-transition metal including lead, zinc, tin, calcium, strontium, barrium, and cadmium (U.S. Pat. No. 4,230,834). U.S. Pat. No. 3,855,040 shows a combination of a peroxy initiator, a strong acid, and ferrocene. Relative to the use of transition metals or transition metal salts as accelerators, U.S. Pat. No. 4,230,834 shows that the use of any transition metal ion accelerator, especially copper, causes poor bonding or low tensile strength values when compared to the non-transition metal ion accelerators disclosed therein.